The present invention relates to a multicarrier type radio transmission apparatus for combining signals of a plurality of channels with a plurality of carrier frequencies into one signal to effect radio transmission.
This application is based on Japanese Patent Application No. 10-240731, filed Aug. 26, 1998, the content of which is incorporated herein by reference.
The individual amplification system was first put into practice as the amplification system in the multicarrier radio transmission apparatus. In the radio transmission apparatus of individual amplification system, amplifiers 10-1, 10-2, . . . , 10-n of a number equal to the number of carrier frequencies (the number of channels) used are provided as shown in FIG. 1 and transmission signals of the respective channels (signals from signal generators 12-1, 12-2, . . . , 12-n) are amplified by the respective amplifiers 10-1, 10-2, . . . , 10-n. Since the signals input to the respective amplifiers 10-1, 10-2, . . . , 10-n are signals each corresponding to one of the channels, there occurs no possibility that the signal of each channel will interfere with the signal of another channel. Thus, the amplifiers 10-1, 10-2, . . . , 10-n can be operated in a high-efficiency operating region. Further, it is advantageous in the heat radiation because the amplifiers 10-1, 10-2, . . . , 10-n are separately provided for n channels.
Output signals of the amplifiers 10-1, 10-2, . . . , 10-n are combined in power by a power combiner 18 and supplied to an antenna (not shown). In order to prevent signals reflected from the power combiner 18 from being fed back to the amplifiers 10-1, 10-2, . . . , 10-n and causing distortion in the amplified signals, it is necessary to insert isolators 16-1, 16-2, . . . , 16-n between the amplifiers 10-1, 10-2, . . . , 10-n and the power combiner 18 so as to maintain isolation between the channels. However, if the isolators 16-1, 16-2, . . . , 16-n are inserted, there occurs a problem that great loss occurs to produce a large amount of heat.
Further, in order to simplify the construction of the power combiner 18, it is necessary to supply the amplified signals to the power combiner 18 via band-pass filters 14-1, 14-2, . . . , 14-n having high channel selectivity. However, since the pass bands are fixed in the conventional filters, it is impossible to change the frequency of the carrier signal of each channel if the pass bands of the filters 14-1, 14-2, . . . , 14-n are set to correspond to the bands of signals of the respective channels output from the signal generators 12-1, 12-2, . . . , 12-n. In the actual transmission system, there is a request for changing the carrier frequency of each channel to a carrier frequency of another channel assigned to the system or there will be a request for changing the bandwidth of the carrier frequency in the future, but the individual amplification system cannot cope with the request. In order to cope with the request at least to some extent, the pass bands of the filters 14-1, 14-2, . . . , 14-n are set equal to each other and the carrier frequency band of all of the channels is set as the pass band. However, in this case, it is also necessary to insert the isolators in order to improve the channel selectivity. Therefore, the power combiner 18 becomes complicated in construction.
In order to solve the problem of the individual amplification system, a collective amplification system was developed. As shown in FIG. 2, in the collective amplification system, signals of carrier frequencies of the respective channels output from the transmission signal generators 12-1, 12-2, . . . , 12-n are first combined by a power combiner 22 and then collectively amplified by an amplifier 24. Thus, since the amplifier 24 is not provided in the succeeding stage of the amplifier 24, it becomes unnecessary to connect an isolator in the preceding stage of the power combiner 22 and a problem of loss and heat generation which occurs in the individual amplification system by the presence of the isolators will not occur. However, since a plurality of channel signals are simultaneously input to the amplifier 24, the linearity of the amplifier 24 becomes important in order to prevent inter-modulation distortion between the channel signals (generally, the high linearity operation and the high-efficiency operation conflict with each other), but in recent years, the high-efficiency operation of a linear amplifier can be attained by various technical improvements. In this respect, the advantage in efficiency of the collective amplification system is recognized.
However, in the collective amplification system, the operation efficiency of approx. 40% at maximum can be attained when the maximum permissible number of channels are received, but if there is an unused channel, the efficiency is lowered. This is because the amplifier 24 must be operated in a low-efficiency operating region (low input power portion) when the number of channels used is small since the input power to the amplifier 24 is changed according to the number of channels used. Further, in the collective amplification system, since heat generation is concentrated in one portion of the amplifier 24, it becomes necessary to take a large-scale heat radiation measure. Since the number of accommodated channels is determined by the maximum permissible number of channels of the amplifier 24 and the value of the maximum permissible power of the filter 26, there occurs a problem that it is difficult to increase the number of accommodated channels after designing of the system. Further, there occurs a problem that large permissible power becomes necessary as the specification of the filter 26 in order to deal with a large number of channels.